This invention relates to a method for the digital transmission of data of the kind in which data is transmitted by a system known as a synchronous digital hierarchy (SDH) in which bits of information are fed in a stream and are multiplexed with bits from other streams in a system known as tributaries which are combined or separated by a multiplexer/demultiplexer (MUX). Such a system deals only with signals in digital form and enables an efficient and noise free transmission of data to take place. Signals transmitted under SDH are the subject of international standards and one particular standard is defined in CCITT recommendations G.707, G.708 and G.709. Unfortunately phase distortion can occur in a transmission network corrupting its SDH and this is due to pointer justification.
A solution to deal with this problem is set out in our co-pending British Patent Application No. 9111235.9. That patent application also describes a method of dealing with the problem in which a data stream at the node of a synchronous SDH transmission network has a specified line frequency and is composed of frames. Each frame is constructed as to contain a reference word and the node of the transmission network has a node frequency. In the improved method of that invention the method stores the incoming data stream in a buffer store at the node and uses a line reference of the incoming data stream to extract a data pointer from the data stream for each frame. This data pointer indicates the location of the reference word of that frame in the buffer store. Furthermore the timing pointer is extracted from the incoming data stream utilising the line reference and a line clock, the frequency of which is a multiple of the line reference. The timing pointer so extracted is converted into a reference value by using the node reference and a node clock. The reference value is used to generate a node timing pointer. The node timing pointer is compared with a read address and data pointer and read-out from the buffer store is justified as a result of the comparison which takes place.
The problem arising with the use of the invention described in our previous application is that the pointer information which was added, failed to pass through apparatus transmitting the frames. This obviously is a disadvantage and it is necessary for the pointer information to pass through without any corruption or distortion. It is an object of the present invention to provide an improved method and apparatus which enables the pointer information to pass cleanly through the network.